Parallel MRI increases image acquisition speed by using spatial sensitivity of an array of receiver coils in addition to the spatial encoding provided by applied linear magnetic field gradients. The additional spatial information enables reduction in the number of acquired phase-encoding (PE) k-space lines needed for conventional image reconstruction and therefore accelerates data acquisition. The increase in image acquisition speed is usually stated in terms of the reduction factor R which, for fixed nominal image resolution and field-of-view (FOV), comprises the ratio of the number of acquired PE lines for a conventional scan to the number of acquired PE lines for a parallel imaging scan. The PE lines which are absent from the parallel imaging data set are termed herein the missing set and the data sampled during accelerated imaging is termed herein the reduced set of PE lines. In addition the union of the reduced set and the missing set are referred to as the nominal set of PE lines which are equivalent to the set of PE lines acquired during conventional imaging since it is the sampling characteristics of this set which determines the nominal digital image resolution and FOV.
Known GRAPPA (GeneRalized Autocalibrating Partially Parallel Acquisitions) parallel imaging does not require explicit knowledge of receive fields for each element of a receiver array. Instead, the GRAPPA method uses data from the receiver array in an auto-calibration procedure which determines a set of synthesis coefficients used to synthesize the missing set of PE lines from the reduced set of PE lines over the set of receiver coils. The autocalibration procedure acquires a set of auto-calibration scan (ACS) lines which are a subset of the nominal PE lines. It is these ACS lines which are used to calculate synthesis coefficients over a set of receiver array channels.
For a GRAPPA EPI (echo planar imaging) time series, the ACS data is often collected on k-space trajectories which sample the data in a segmented interleaved fashion, as depicted in FIG. 1 for R=2. FIG. 1 shows two segments of a 2-shot interleaved ACS k-space trajectory having a vertical phase-encoding direction and a horizontal frequency-encoding direction. In FIG. 1 the total number of ACS lines Na is not necessarily equal to the nominal matrix size in the PE direction. Further, interleaved EPI ACS is sensitive to motion corruption. If motion (e.g., due to respiration, heart rate or patient movement, for example) occurs between the sampling of the interleaved ACS segments, artifacts, most notably residual aliasing, occur in GRAPPA-reconstructed EPI images. A system according to the present invention addresses this deficiency and related problems.